1. Field of the Invention
The present invention relates generally to measuring devices for making linear and angular measurements. More particularly, it relates to measurement of small-angle or small displacement position using capacitive measurement techniques, while exhibiting a reduced sensitivity to motions that are not in the direction or axis of the desired measurement.
2. Background of the Invention
Numerous capacitance-type measuring devices for making linear and angular measurements have been developed. For example, U.S. Pat. No. 4,972,725 to Choisnet describes a capacitive sensor of a torsion angle and a torque or moment measuring instrument provided with a sensor. Similarly, U.S. Pat. No. 5,657,006 to Kinoshita et al. describes a rotation angle sensor that detects a rotation angle by amounts of changes in electrostatic capacitance.
Kinoshita et al. disclose in its FIG. 1 a sensor having two fixed plate electrodes, which are opposed to each other, and a rotational plate inserted between the two fixed electrodes. The first fixed electrode is split into two members, each member having a semicircular shape and the members not conducting with each other. The second fixed electrode is split into two members, each member having a semicircular shape and the members not conducting with each other. Finally, the rotational plate electrode is also split into two members, each member having a semicircular shape and the members not conducting with each other. The rotational plate split members are bonded to the shaft. The first fixed electrode emits a signal that is measured by the second fixed electrode via capacitance. Particularly, the rotational plate acts as a shield changing the capacitance valued measured by the second fixed electrode members.
For example, in Kinoshita's sensor, the shaft is rotated in correspondence to a rotation angle to be detected. In response to this rotation of the shaft, the rotational plate rotates to form electrostatic capacitances between the first member of the first fixed electrode and the first member of the second fixed electrode, between the first member of the first fixed electrode and the second member of the second fixed electrode, between the second member of the first fixed electrode and the first member of the second fixed electrode, and between the second member of the first fixed electrode and the second member of the second fixed electrode. The differential capacitance between the two members of the second electrode changes as the rotational plate overlaps different portions of the members, allowing an angular position of the plate to be detected.
Having only two members in each plate allows the angle sensor to operate over a large range of motion. Indeed, these and other conventional angle sensors are designed for wide angle positioning over a large range of operation. As such, their resolution and gain errors due to motion in the non-measuring direction are unsuitable for use in small angle measurement devices.
As suggested above, gain error often occurs in traditional capacitive angle sensors due to non-angular displacement. For example, as temperature increases, a member of the rotating electrode typically shifts to a member in one of the fixed electrode plates. Such small movement in the non-axial direction will be inaccurately reported by the sensor as a change in the angular direction. Particularly, because the rotor elements of an active rotor array have conductive surfaces, motion of the rotary electrode that is not along the primary measurement path can introduce additional capacitances that are parasitic to the function of the sensor, causing an error in gain and a reduced sensitivity that is unsuitable for small angle measurement.
In conventional angle measurement sensors, the rotating or movable electrode of an array system is attached to ground or is left floating. FIGS. 1 and 2 describe the resulting parasitic configurations in a conventional array, such as the array of Kinoshita et al. described above, for both arrangements of the rotating electrode plate.
FIG. 1 is a schematic diagram of a capacitive bridge 100 formed by elements of a conventional angle sensor when the rotating plate is electrically isolated and floating. The bridge includes nodes A to D and capacitors AC 110, AD 120, BD 130, BC 140, AE 150, BE 160, CE 170 and DE 180.
Node A represents the first member of the first fixed electrode. Similarly, node B represents the second member of the first fixed electrode. Node C represents the first member of the second fixed electrode. Finally, node D represents the second member of the second fixed electrode.
Capacitors AC 110, AD 120, BD 130 and BC 140 represent the various capacitances formed between each node, as discussed above. For example, AC 110 represents the capacitance between nodes A and C. AD 120 represents the capacitance between nodes A and D. BD 130 represents the capacitance between nodes B and D, and BC 140 represents the capacitance between nodes B and D.
Finally, effective capacitors AE 150, BE 160, CE 170 and DE 180 represent each of the capacitances formed between each of the nodes and node E, a floating point. The four capacitors AE 150, BE 160, CE 170 and DE 180 form an effective AC grounding point at node E. When the rotating electrode experiences an axial shift, the values of capacitors AE 150 and BE 160 increase, while the values of capacitors CE 170 and DE 180 decrease.
Nodes A and B, representative of members of a first fixed electrode that emits a signal to the second fixed electrode, are typically low impedance, and will remain relatively unaffected by changes to capacitors AE 150 and BE 160. However, nodes C and D, representative of members of a second fixed electrode that measures capacitance, are high-impedance nodes that are extremely sensitive to capacitive loading. Hence, the variable capacitive load of capacitors CE 170 and DE 180 causes a gain change in the bridge due to loading effects.
FIG. 2 is a schematic diagram of a capacitive bridge formed by the elements of a conventional angle sensor when the rotating plate is grounded. Similar to FIG. 1, the bridge includes nodes A to D and capacitors AC 110, AD 120, BD 130, and BC 140. In addition, the bridge includes capacitors AG 210, BG 220, CG 230 and DG 240 connected between each of the nodes and ground. The system shown in FIG. 2 is simpler than FIG. 1, but the net result is the same. If the rotor changes axial position, this is reported as a gain change in the bridge measurement.
Another challenge in obtaining accurate, linear, stable and repeatable results in a position sensor that operates over a small range of motion with a high resolution of position lies with the limited range of capacitive rotary sensors. Capacitive rotary sensors typically have somewhat less than one-half of one rotation of full-scale range, thereby limiting accuracy and resolution during small angle measurements of microradians of motion or smaller.
Yet another challenge with capacitive sensors lies with the non-linear position signal produced by plate type capacitive sensors used for linear positioning. In plate-type sensors where the gap of the capacitor is varied to effect capacitance, the position signal is not typically linear, and must be corrected to produce a linear signal.
Thus, it is desirable to create small-angle or small-displacement capacitive sensors that have greatly reduced sensitivity to typical sources of mechanical positioning error. Some applications for such sensors include a motor position sensor for a linear or rotary actuator with a small range of motion. For example, a magnetic or piezo motor may include such a sensor. Other applications include a linear or rotary sensor used with a mechanical spring for measuring force or torque, a sensor in a position servo having a magnetic motor as a magnetic force sensor, and a sensor for measurement of position in micro-positioning X/Y or rotary platform. More particularly, a rheology, weighing or other load cell application may use the sensor in construction of force or torque sensor. An atomic force microscopy application may use the sensor for position control or force measurement. Finally, a small motion mechanical servo system may use the sensor in a precision position indicator.